Polypropylene exhibits several advantageous properties, for example, relatively low density, excellent resistance to higher temperatures and aqueous and non-aqueous liquids. Polypropylene also has the less favorable characteristic of inadequate impact strength at temperatures below room temperature especially below 0.degree. C. Adequate impact strength, however, is required and is of importance in many uses such as for example freight containers, suitcases, automobile parts and similar parts. Articles made of high density polyethylene possess this satisfactorily high impact strength but show a lower resistance to deformation at high temperatures. Ethylene propylene elastomers, saturated as well as unsaturated, exhibit good mechanical properties such as high heat aging resistance, high ozone resistance and impact resistance at low temperatures, such that the copolymers are also excellently suited for use where the product is exposed to weathering. Due to the elastomeric properties of these copolymers, it is popular to combine elastomers with harder polymers. Many attempts have been made to combine the properties of polypropylene and/or polyethylene and/or ethylene propylene rubbers by the production of binary or ternary blends or molding compositions. Mixtures of polypropylene and polyethylene are known, however, these mixtures exhibit an impact strength at room temperature which is hardly improved over polypropylene itself. Also, blends of isotactic polypropylene and ethylene propylene elastomers, which are cross-linked or partially cross-linked by vulcanizing agents, require improvement with respect to tensile strength, modulus, and hardness values attainable.
Blends of polyethylene with EPDM terpolymers of ethylene, propylene and nonconjugated diene are known from U.S. Pat. No. 3,919,358 and exhibit high tear strengths. The blends do not show sufficient heat resistance due to the low melting temperature polyethylene. Ternary molding compositions are also described in British patent No. 1,154,447. These crystalline polypropylene, polyethylene and ethylene propylene block copolymer ternary blends exhibit a tensile strength of only 10 N/mm.sup.2 and are not flexible at temperatures below 30.degree. C.
Various polyolefin ternary blends have still been used in many industrial applications because of the balance achieved among several properties, for example, between rigidity and impact resistance. In particular, ternary blends known as thermoplastic elastomers (TPE's) are used where rigidity and impact resistance are important. A thermoplastic olefin (TPO) is a subset of TPE's. Typically a TPO is a blended product of a hard segment of semi-crystalline polypropylene or semi-crystalline polyethylene and a soft segment of an olefin elastomer such as ethylene propylene rubber (ER), ethylene/propylene/diene terpolymer (EPDM), polyisobutylene or polybutadiene. The TPO may also be subject to partial cross-linking in order to improve physical properties. In recent years however, the required merits of the blend such as good moldability, appearance of the molded articles i.e. high gloss, paintability, thermal resistance, low temperature impact resistance and the property that the deformation caused by impact is easily restored have become more important. The conventional TPE's and TPO's have not always met these requirements.
U.S. Pat. No. 4,748,206 to Nogiwa discloses quaternary blends of polypropylene, ethylene propylene rubber, ULDPE and LLDPE or MDPE and addresses some of these issues. (ULDPE=ultra low density polyethylene, LLDPE=linear low density polyethylene and MDPE=medium density polyethylene) However, the inclusion of 5 to 30 wt. % MDPE and 5 to 50 wt. % ULDPE is limiting. Spielau et al. in U.S. Pat. Nos. 4,319,004 and 4,319,005 disclose ternary blends of 38 to 48 wt. % non-amorphous ethylene propylene copolymer (at least 65 wt. % ethylene), polyethylene, preferably high density polyethylene (HDPE) and 3 to 30 wt. % non-amorphous propylene homopolymer or (polypropylene copolymer up to 12 wt. % ethylene).
Further U.S. Pat. No. 4,822,855 to Kobayashi discloses a blend of 30 to 70 parts polyethylene, 30 to 70 parts of random propylene copolymer of up to 12 wt. % ethylene and 70 to 200 parts ethylene propylene rubber. Kobayashi, however, uses very low density polyethylene with a density between 0.86 to 0.91 g/cm.sup.3.
There still exists a need in the an to develop a rubber toughened thermoplastic olefin for use in molded applications that has high stiffness and does not exhibit low temperature brittleness.